A muconic acid derivative isomeric organic-inorganic metal halide with high-temperature phase transition

Organic-inorganic metal halides (OIMHs) have attracted immense attention in recent decades due to their diverse physical properties, such as optoelectronic properties, photoluminescence, magnetism, and ferroelectricity. Besides these fascinating physical properties, OIMHs have also received much interest for their phase transitions. Muconic acid has captured extensive interest owing to the presence of cis,cis-form (Z,Z) and trans,trans-form (E,E) isomers. However, there have been no reports on muconic acid derivative OIMHs with phase transitions. In this work, we successfully synthesized the first muconic acid derivative OIMHs (Z,Z)- and (E,E)-[BPHD]ZnCl4 (BPHD = N1,N6-bis(piperidine-1-yl) hexa-2,4-diene diamide). Both of them crystallized in the monoclinic space group P21/c at room temperature. Notably, (Z,Z)-[BPHD]ZnCl4 does not exhibit phase transition behavior, while (E,E)-[BPHD]ZnCl4 has a high-temperature phase transition of up to 449.4 K, which is higher than that of the majority of OIMH phase transition compounds. Furthermore, the high-temperature structural analysis of (E,E)-[BPHD]ZnCl4 discloses that its high-temperature phase transition originates from the order-disorder transition of the piperidine groups of one (E,E)-[BPHD]2+ cation and the swing motion of the piperidine groups of another (E,E)-[BPHD]2+ cation. This work expands the family of OIMHs and provides a new platform for designing high-temperature phase transition OIMHs. (Z,Z)- and (E,E)-[BPHD]ZnCl4 are the first muconic acid derivative organic-inorganic metal halides, in which the EE-isomer exhibits a high phase transition temperature of up to 449.4 K.